The present invention relates to hydrocarbon cracking catalysts, and more specifically to improved zeolite cracking catalyst compositions which are capable of producing a high yield of aromatic and olefinic gasoline fractions of enhanced octane rating.
It is generally known that hydrocarbon cracking catalysts which are promoted with stabilized zeolites, particularly ion exchanged synthetic faujasite (X and Y-type), are capable of producing high yields of gasoline fractions from petroleum feedstocks such as gas oil. These cracked gasoline fractions are subsequently combined with octane enhancing additives such as tetraethyl lead to produce high octane motor fuel.
Recent emphasis on air pollution control has dictated a need for removing metal-type octane enhancing additives from commercial gasolines. To produce non-leaded gasoline of sufficient octane rating for use in modern automobile engines it is generally necessary for the refiner to use a blend of petroleum gasoline fractions which are of inherently high octane rating. Highly aromatic and olefinic gasoline fractions are of particular use to the refiner. Unfortunately, however, gasoline fractions produced by the cat-cracking of gas-oil using crystalline zeolite type cracking catalysts are of relatively low aromatic and olefinic content. Accordingly, relatively expensive subsequent catalytic treatment is generally required to increase the aromatic and olefinic content, and hence the octane rating of cat-crack gasolines, to thereby avoid the addition of metal type octane enhancing additives.
We have found that catalytically cracked gasoline fractions having increased aromatic and olefinic content can be produced by contacting hydrocarbon feedstocks with an improved exchanged zeolite/amorphous matrix type cracking catalyst.
Broadly, our invention contemplates a zeolite cracking catalyst which comprises a Y-type zeolite from which the sodium content has been removed in a novel manner and replaced with hydrogen ions. Calcination at 1000.degree. F. prior to removal of all Na.sub.2 O enables the H-Y sieve to yield a higher octane gasoline product.
More specifically, we have made the finding that a Y-type zeolite having a silica to alumina ratio in excess of about 3.0 which has been ion exchanged with hydrogen ions in a novel manner will catalyze the cracking or petroluem to produce gasoline fractions of unusually high aromatic and olefin content. In addition a catalyst promoted with this sieve produces high yields of C.sub.4 olefins without greatly changing gasoline selectivity or coke and hydrogen yields. The production of high level yields of C.sub.3 and C.sub.4 olefins can be enhanced even more by the addition of promotional amounts of calcined rare earth exchanged zeolite (CREY) to the catalyst composite. The primary purpose of CREY addition is, however, to enhance activity.
To produce the hydrogen exchanged Y-type zeolite, i.e., H-Y contemplated herein, the following procedure is used:
1. A sodium type Y zeolite is exchanged with an acidified ammonium salt solution having a pH of approximately 3.0 to 3.5 until the zeolite contains less than 4% and preferably about 3 weight percent Na.sub.2 O. PA1 2. the ammonium exchanged zeolite is then heated (calcined) at about 1000.degree. to 1500.degree. F. for 0.1 to 12 hours. PA1 3. The calcined zeolite is again exchanged with ammonium ions to lower the Na.sub.2 O content of the zeolite to below 1% by weight, PA1 4. The exchanged zeolite is calcined at 1000.degree. to 1500.degree. F. for 0.1 to 12 hours, cooled & recovered.
The particularly significant point in our novel method of preparation is the first exchange step with an acidified ammonium salt solution. Prior art preparations of hydrogen exchanged zeolite involve the use of ammonium salt solution having a pH in the range of 4.0 to 7.0.
The first step in our process is the selection of a suitable zeolite. Our product can be prepared from certain natural and synthetic zeolites, such as the natural and synthetic faujasites. A particularly suitable raw material is synthetic Y-type faujasite in the sodium form having the formula: EQU 0.9 .+-. 0.2 Na.sub.2 O:Al.sub.2 O.sub.3 :WSiO.sub.2 :XH.sub.2 O
wherein W has a value of greater than 3 and up about 6 and X may have a value as high as 9. This material is disclosed in U.S. Pat. No. 3,130,007 to Breck.
The next step of our process is the exchange of the above zeolite with an acidified ammonium salt, amine salt or other salt which on calcination decomposes and leaves an appreciable portion of the zeolite in the hydrogen form. The acidification can be accomplished by adding a mineral acid to the exchange solution until a pH in the range of 3.0 to 3.5 is reached. Examples of suitable compounds for the exchange procedure include ammonium chloride, ammonium sulfate, tetraethyl ammonium chloride, tetraethyl ammonium sulfate, etc. Ammonium salts because of their ready availability and low cost, are the preferred reagents of this step of the reaction. The preferred combination of ammonium compound and acid is ammonium sulfate and sulfuric acid due to the ease with which sulfate ions can be washed from the zeolite. Since acids themselves can exchange hydrogen ions into the zeolite structure a sulfuric acid solution alone having a pH of from 3.0 to 3.5 is also contemplated as the exchange medium for this step. It will be understood that to preclude the formation of unwanted side products the ammonium compound and acid will have the same cation (ammonium chloride-hydrochloric acid).
This first exchange is carried out rapidly with an excess of salt solution. The salt may be present in excess of about 5 to 600%, preferably about 20 to 300%. The exchange is generally completed in a period of about 0.1 to 24 hours. This preliminary exchange reduces the alkali metal content of the zeolite. Chemical analysis of the product at this stage shows that the product contains 1.5 to 4% by weight of alkali metal. It is preferred that the Na.sub.2 O content of the product at this stage be no more than 1.5 to 2.9 weight percent alkali metal oxide.
After the preliminary exchange is completed, the zeolite is filtered, washed and dried. It is desirable that the zeolite be washed sulfate free at this stage of the process. The composite is then calcined at a temperature in the range of 1000.degree.-1500.degree. F. The calcination is preferably carried out for a period of 0.1 to 12 hours at a temperature of from 1200.degree.-1500.degree. F. The calcination seems to cause an internal rearrangement or transfer so that the remaining alkali metal ions are lifted from their buried sites and can now be easily ion exchanged in the next step by a mild treatment with a salt solution.
In the next step of the process, the zeolite is treated with a solution of ammonium salt or amine salt, etc. This exchange should be carried out for a period of 0.1 to 24 hours, conveniently for a period of 3 hours. It is desirable that this exchange be conducted at a temperature of 25.degree. to 150.degree. C. At the end of this time the material is again filtered, washed thoroughly to remove all traces of sulfate and stabilized by calcination at high temperatures. It is preferable that the alkali metal oxide content of the product be no more than 0.5 weight percent prior to the final stabilization step.
The product should not be dried and then stored for long periods of time prior to the final stabilization. It can be dried immediately prior to stabilization in a preheat zone of the furnace. However, if the material is to be held for any appreciable period of time, it should be held as a wet filter cake (60% total volatiles) and can be held in this condition for a period of more than 48 hours without damage. The final stabilization is achieved by heating the material to a temperature of 1100.degree.-1500.degree. F. for a period of 0.1 to 12 hours.
The catalysts contemplated in the present invention may comprise either essentially 100% H-Y or the H-Y may be admixed with an inorganic oxide component. Where it is desired to obtain a catalyst in particle form, the precursor synthetic sodium Y zeolite ingredient may be in the form of a formed fluidizable or pelleted particle. This fluidizable or pelleted particle is subjected to the above exchange and calcination procedures to obtain a catalyst product which is essentially all H-Y.
When it is desired to obtain a catalyst which contains H-Y admixed with an inorganic oxide matrix, the H-Y in finely divided form is blended with a suitable inorganic oxide matrix component. Suitable matrix components are generally described in the prior art, and may be selected from inorganic hydrogels such as silica, alumina and silica-alumina hydrogel. In general, these hydrogel components are essentially amorphous and are readily blended with the highly crystalline H-Y component. It is also contemplated that matrix components such as clay and combinations of clay with amorphous inorganic hydrogels may be combined with the presently contemplated H-Y catalyst component.
As indicated above, the catalysts contemplated herein may be obtained in fluid form wherein finely divided spray dried microspheres having a particle size range of from about 10 to 105 microns are obtained using conventional catalyst preparation techniques. It is also contemplated that catalysts suitable for moving or fix bed operation wherein the catalyst particle size ranging from about 5 to 40 mesh may also be obtained using standard catalyst preparation procedures.
The present catalysts are used in the cat-cracking of heavy petroleum feedstocks. Preferably the catalysts are used in a conventional manner wherein the petroleum feedstock such as gas oil is contacted with a bed of the catalyst at a temperature of from about 800.degree. to 1000.degree. F. The contact time of the petroleum feedstock with the catalyst will depend on the specific catalytic process employed; however, it is found that contact times as low as 5 seconds up to 300 seconds may be advantageously employed using the highly active catalyst of the present invention.
When the present catalyst is used in combination with a typical semi-synthetic matrix, i.e., a matrix comprising silica-alumina hydrogen and clay, wherein the H-Y component is present in amounts ranging from about 5 to 50% by weight, the catalyst will exhibit a microactivity on the order of from about 45 to 80 when tested under standard conditions. As was mentioned previously, H-Y can also be combined with portions of calcined rare earth exchanged Y-type zeolite designated CREY the preparation of which is disclosed in U.S. Pat. No. 3,402,996, example VI in particular. When the zeolite component contains both H-Y and CREY the preferred ranges are; 5.0 to 50 weight percent H-Y, 1.0 to 10 weight percent CREY and 40 to 90 weight percent inorganic matrix. The addition of small amounts of CREY further enhances the yields of C.sub.3 and C.sub.4 olefins in the product stream. The combination H-Y/CREY promoted catalyst will exhibit a microactivity on the order of from about 50 to 80 when tested under standard conditions. The product distribution obtained using the present catalysts reveals that high yields of gasoline fractions (C.sub.4 through C.sub.9) are obtained. These gasoline fractions possess an extraordinarily high aromatic content which is readily measured using standard NMR techniques. It is also found that the catalysts produce exceptionally high yields of C.sub.4 olefins and branch chain C.sub.4 isomers. The present catalysts produce a cracked product which contains only small quantities of undesirable dry gas (hydrogen, C.sub.1 and C.sub.2) products and only small amounts of coke.